1. Field of Invention
This invention relates to a method to produce extruded alumina (Al.sub.2 O.sub.3) bodies containing at least 99 weight % (w/o) Al.sub.2 O.sub.3 with less than 1 w/o ZrO.sub.2 having high flexural strength.
2. Description of the Previously Published Art
Zirconia has been added to alumina to improve its properties.
In U.S. Pat. No. 4,316,964 ZrO.sub.2 was added in an amount from 5 to 95% of the volume of the alumina ceramic to increase the fracture toughness and strength of the Al.sub.2 O.sub.3 /ZrO.sub.2 ceramic. The zirconia had dissolved in it a rare earth oxide such as Y.sub.2 O.sub.3, CeO.sub.2, La.sub.2 O.sub.3 and/or Er.sub.2 O.sub.3 to promote retention of the metastable tetragonal ZrO.sub.2. The 5 volume % zirconia in the above composition corresponds to 7.3% on a weight basis.
U.S. Pat. No. 4,419,311 discloses the production of high-strength ceramic bodies of alumina with unstabilized zirconia for the formation of controlled microfissures. An unstabilized zirconia dioxide and/or hafnium dioxide with a particle size of 0.3 to 1.25 microns was added as a ceramic embedment material to generate tensile stresses for the formation of controlled microfissures. The amount of the ceramic embedment was from 4 to 25% of the volume of the body to be formed. In this composition, 4 volume % corresponds to 5.9 weight %.
U.S. Pat. No. 4,218,253 discloses improving the ductility of sintered alumina bodies by uniformly distributing individual particles of tetragonal zirconia or hafnia in amounts of 1%-50% by volume. The dispersed particles are to have an average size of 0.05 to 2 microns. In this composition 1 volume % ZrO.sub.2 is equal to 1.5 weight % ZrO.sub.2. In this reference, and in the above described reference, there is no mention of extruded articles. Only articles prepared by pressing techniques are disclosed.
S. Hori et al in their article "Influence with Small ZrO.sub.2 Additions on the Microstructure and Mechanical Properties of Al.sub.2 O.sub.3 " in Advances in Ceramics, Vol. 24A: Science and Technology of Zirconia III, 1988 p. 423 (published by American Ceramic Society) disclose that the addition of ZrO.sub.2 in an amount of up to 1 wt % increased the strength but decreased the toughness of Al.sub.2 O.sub.3 due to the strong dependence of these properties on the Al.sub.2 O.sub.3 grain size. The articles produced have a three point flexural strength of 58,000 psi at 1 w/o ZrO.sub.2 levels. This value is substantially below our average value of 73,000 psi (obtained without organic binders). Furthermore, the products of this reference were prepared by isostatically pressing mixtures of alumina and alumina/zirconia powders, whereas we are mixing alumina powder with a zirconia sol, thereby introducing the zirconia at a much finer particle size (5-10 nanometers). The zirconia, with particles this small, permit more intimate mixing of the zirconia with the alumina.
P. F. Becher in "Transient Thermal Stress Behavior in ZrO.sub.2 -Toughened Al.sub.2 O.sub.3 " in the Journal of the American Ceramic Society, 64, No. 1, January 1981, pp. 37-39, discloses characteristics of hot-pressed Al.sub.2 O.sub.3 -ZrO.sub.2 composites. The article discusses how the resistance of ceramics to transient thermal stresses can be improved by modifying the thermal-mechanical properties of the matrix. The articles produced have 61,000 psi (3 point flexural strength) at 0.75 w/o ZrO.sub.2 levels. These strengths are again lower than our values (without binder). The composites were prepared by mixing Al.sub.2 O.sub.3 and ZrO.sub.2 obtained from mixtures of gelled alkoxide derived sols. The powders were vacuum hot pressed in order to obtain dense products. This procedure is typical for gel derived powders, which are typically difficult to densify. Our extruded product, densifies very readily.
3. Objects of the Invention
It is an object of this invention to extrude Al.sub.2 O.sub.3 bodies containing at least 99 w/o Al.sub.2 O.sub.3 and less than 1 w/o ZrO.sub.2 having high flexural strength.
It is a further object of this invention to form extruded Al.sub.2 O.sub.3 bodies by combining an organic dispersing agent and a zirconia sol to produce high-strength bodies.
It is a further object of this invention to form extruded Al.sub.2 O.sub.3 bodies by combining an organic dispersing agent, a zirconia sol and an organic deaerating agent to produce high-strength bodies.
It is a further object of this invention to combine an organic dispersing agent, a zirconia sol and an organic binder with Al.sub.2 O.sub.3 to produce large size, high-strength bodies.
It is a further object of this invention to form extruded Al.sub.2 O.sub.3 having less than 1 w/o ZrO.sub.2 bodies having no organic binder and having 3 point flexural strengths greater than 65,000 psi.
It is a further object of this invention to form extruded Al.sub.2 O.sub.3 bodies having less than 1 w/o ZrO.sub.2 using organic binders with 3 point flexural strengths greater than 55,000 psi.
These and further objects will become apparent as the description of the invention proceeds.